The presently disclosed subject matter relates to processes for preparing nanocrystals, including processes for preparing core-shell nanocrystals.
Synthetic advances have improved understanding of quantum confined metal chalcogenide semiconductor nanocrystals, known as quantum dots (QD or QDs). Scalability of synthesis, control over nanocrystal size, control over distribution of nanocrystal size, and control of photoluminescence quantum yield have all improved in recent years. However, many of these synthetic advances have been achieved by empirical optimization because nanocrystal formation can be a complex process that is controlled by many interdependent variables. This difficulty can be made worse by a limited collection of chalcogen precursors, many of which are pyrophoric, toxic, difficult to purify, too reactive, or too unreactive. Such issues can limit synthetic reproducibility and create nanocrystals with ill-defined chemical compositions. Improved precursors are thus broadly important to nanocrystal science, both because they can provide access to materials with optimized optoelectronic properties and because they can increase understanding of crystallization mechanisms and nanocrystal structure.
One subclass of quantum confined metal chalcogenide nanocrystals is core-shell nanocrystals, which can be used as phosphors for lighting, given that their size and interfacial composition can be precisely controlled to optimize their luminescence wavelength linewidth, quantum yield and their photostability. To achieve the performance needed for certain on-chip lighting applications, these materials should withstand high operating temperatures (e.g., 150° C.) and intense illumination fluxes (e.g., 200 W/cm2) that can lead to multiexciton Auger recombination and photochemical degradation. High performance materials can have graded alloy compositions that serve two purposes: they reduce the multiexciton Auger recombination rates and minimize interfacial strain, thereby allowing the conformity and stability of shelling layers. However, the structure of high performance interfaces can be difficult to control.
Among classes of colloidal crystallizations, semiconductor quantum dots are thought to form via homogeneous nucleation and growth mechanisms proposed by La Mer, a three-phase mechanism shown in FIG. 1. Monomers (ME) are generated by a slow reaction between a metal salt (MX2) and a chalcogen precursor (ER2) that can limit subsequent crystallization (FIG. 2). FIG. 2 is a scheme presenting a generalized mechanism of precursor-limited homogeneous nucleation and growth of nanocrystals. The precursor conversion rate can play a role in the kinetics of nanocrystal nucleation and growth by determining the kinetics of monomer supply to the crystallization medium. The monomer supply rate during the nucleation phase can control the number of nanocrystals produced and therefore can be used to control the final size and size distribution.
Examples of sulfur precursors include phosphine sulfides (R3P═S), bis-trimethylsilyl sulfide ((TMS)2S), and alkyldithiocarbamates. Hydrogen sulfide (HS), can also serve as a sulfur precursor. Hydrogen sulfide can be produced by heating elemental sulfur in alkane and/or amine solvents. For example, elemental sulfur can be reduced in the presence of alkylamines and 1-octadecene producing soluble sources of hydrogen sulfide, which can then be used to prepare metal sulfides.
(TMS)2S reacts rapidly with metal salts at temperatures near room temperature (about 20-25° C.), leading to reactions that can be complicated by the kinetics of injection. These complications can limit reaction scale and reproducibility. R3P═S derivatives can react sluggishly above 300° C. and produce low reaction yields. Reactions of elemental sulfur with alkanes and amines, which generate hydrogen sulfide, can be used at intermediate temperatures. However, reactions of elemental sulfur with alkanes and amines generally follow ill-defined radical pathways that can be difficult to control, can be sensitive to the presence of impurities, and can produce sulfur-containing byproducts that can have detrimental effects on nanocrystal properties and limit atom economy. These precursors can suffer from other drawbacks, such as being air-sensitive, producing toxic and noxious hydrogen sulfide, and producing unreliable results.
There are certain selenium-containing precursor compounds known in the art. Examples of selenium precursors include elemental selenium, selenium dioxide, trialkylphosphine selenides and diphenylphosphine selenide. Existing selenium precursors can suffer from many drawbacks similar to those associated with sulfur precursors, with additional complications arising from increased air sensitivity. Phosphine selenide precursor reactivity can be dominated by impurities, and pure phosphine selenide precursors can fail to react quantitatively to provide metal selenide nanocrystals. Reactions of elemental selenium or selenium dioxide with alkanes, alkenes, and/or amines can generate hydrogen selenide in situ, but they can follow poorly defined pathways and generate byproducts that are difficult to control. Such byproducts can reduce atom economy and lead to irreproducibility and impaired nanocrystal properties.
Scalability and reproducibility can be a challenge in preparation of core-shell nanoparticles. One route to core-shell nanoparticles is contacting a core nanocrystal with a mixture that contains a metal salt and a sulfur- or selenium-containing precursor compound, a “shelling” process that will build a metal sulfide or metal selenide shell around the core nanocrystal. However, certain precursor compounds have unpredictable kinetics, and can present challenges that are magnified as the scale of the synthetic batch is increased.
Nanocrystals under 4 nm can be difficult to synthesize with current methods, despite interest in their near-infrared absorption and luminescence arising from strong quantum confinement. Existing procedures can afford poor conversion.
Thus there remains a general need in the art for improved techniques relating to the synthesis of high performance semiconductor nanoparticles and quantum dots, including core-shell architectures.